Power converter

Abstract

The embodiment of the invention provides a power converter which comprises a controller, at least one inverter bridge arm and at least one relay, and the output end of each inverter bridge arm is connected with a power grid through the at least one relay. And the controller is used for controlling the at least one inverter bridge arm to convert the received direct current into alternating current and output the alternating current. And the controller is used for inputting a common-mode voltage signal to the at least one inverter bridge arm to change the voltage instantaneous value of the alternating current output by the at least one inverter bridge arm in the process of controlling the at least one inverter bridge arm to convert the received direct current into the alternating current and output the alternating current. The controller is further used for controlling the relay connected with any inverter bridge arm in the at least one inverter bridge arm to be closed within a set time period in the process of inputting the common-mode voltage signal to the at least one inverter bridge arm, and the absolute value of the instantaneous value of the common-mode voltage signal within the set time period is larger than or equal to a set threshold value.

Description

Technical Field

[0001] This application relates to the field of power electronics, and in particular, to a power converter. Background Art

[0002] Most of the grid-connected relay contacts in inverters adopt the process of plating inert metals on their surfaces. During the operation of the inverter, there may be corrosive gases outside the inverter and enter the interior of the inverter, or there may be some strongly chemically corrosive elements inside the inverter. The above-mentioned corrosive gases and strongly chemically corrosive elements may cause the plated inert metals on the grid-connected relay contacts to oxidize, resulting in an increase in the impedance of the grid-connected relay contacts. Therefore, it is easier to generate heat when the power current passes through, posing a risk of overheating for the inverter. Summary of the Invention

[0003] An embodiment of this application provides a power converter, which can reduce the contact impedance of the oxidation relay in the power converter.

[0004] In a first aspect, this application provides a power converter. The power converter includes a controller, at least one inverter bridge arm, and at least one relay. The output end of each inverter bridge arm is connected to the power grid through at least one relay. The controller is configured to control at least one inverter bridge arm to convert the received direct current into alternating current and output it. The controller is configured to input a common-mode voltage signal to at least one inverter bridge arm during the process of controlling at least one inverter bridge arm to convert the received direct current into alternating current and output it, so as to change the instantaneous value of the voltage of the alternating current output by at least one inverter bridge arm. The controller is further configured to control the closing of the relay connected to any one of the at least one inverter bridge arm during a set period in the process of inputting the common-mode voltage signal to at least one inverter bridge arm, where the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to a set threshold during the set period.

[0005] In this application, during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the relay when it is closed, and the voltage difference across the relay is proportional to the absolute value of the instantaneous value of the common-mode voltage signal input by the controller. Therefore, during the differential pressure cleaning process of the oxidized relay in the power converter by the controller, the controller controls the oxidized relay to close during a set period when inputting the common-mode voltage signal to at least one inverter bridge arm. Since the absolute value of the instantaneous value of the common-mode voltage signal input during the set period is greater than or equal to the set threshold, the common-mode voltage signal will not cross the zero point during the set period. Then the input common-mode voltage signal can adjust the voltage magnitude at the output end of the inverter bridge arm corresponding to the oxidized relay, and the voltage difference across the relay satisfies the differential pressure cleaning requirement during the set period, avoiding the voltage difference across the relay being too low when the relay actually closes due to the conduction delay of the relay, improving the reliability of the differential pressure cleaning of the relay, reducing the contact impedance of the relay, and preventing the power current from passing through the oxidized relay and causing the power converter to overheat.

[0006] In a possible implementation, the controller is configured to control the relay connected to any inverter bridge arm to close when the absolute value of the instantaneous value of the common-mode voltage signal is equal to the set threshold. Here, since the absolute value of the instantaneous value of the common-mode voltage signal is equal to the set threshold during the set period, when there is a conduction delay in the relay, the above set period can cover the time interval from when the controller controls the relay to close to when the relay actually closes, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal is equal to the set threshold, and the common-mode voltage signal can adjust the voltage magnitude at the output end of the inverter bridge arm corresponding to the oxidized relay, making the voltage difference across the oxidized relay satisfy the differential pressure cleaning requirement, and further improving the reliability of the differential pressure cleaning of the relay.

[0007] In a possible implementation, the controller is configured to control the relay connected to any inverter bridge arm to close after the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration, and the set waiting duration is less than or equal to the duration of the set period. Here, if the conduction delay of the relay is long, for example, the time interval from when the controller controls the relay to close to when the relay actually closes is equal to the above set period, then by controlling the oxidized relay to close after the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to just equal to the set threshold and with an interval of the set waiting duration, the actual closing moment of the relay is between another set period, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal is equal to the set threshold, and the common-mode voltage signal can adjust the voltage magnitude at the output end of the corresponding inverter bridge arm, making the voltage difference across the oxidized relay satisfy the differential pressure cleaning requirement, and further improving the reliability of the differential pressure cleaning of the relay.

[0008] In a possible implementation, the common-mode voltage signal is an alternating current signal with periodic variation. The controller is configured to, each time the absolute value of the instantaneous value of the common-mode voltage signal rises from less than a set threshold to greater than or equal to the set threshold and after a set waiting duration has elapsed, control a relay connected to any inverter leg to close once. Wherein, the set waiting duration increases each time the controller controls the relay connected to any inverter leg to close, and the set waiting duration is less than or equal to the duration of the set time period. By setting the set waiting duration to increase each time the controller controls the relay connected to any inverter leg to close, such that the moment when the controller controls the relay to close is at a different position in each set time period, it is possible to avoid the problem that the common-mode voltage signal is at zero at the moment when the relay actually closes due to the uncertain turn-on delay of the relay, ensure that the absolute value of the instantaneous value of the common-mode voltage signal is equal to the set threshold when the relay actually closes, and ensure that the voltage difference across the relay meets the voltage difference cleaning requirement.

[0009] In a possible implementation, after the controller controls the relay connected to any inverter leg to close, the controller is configured to, if the contact impedance of the relay connected to any inverter leg is greater than the set impedance threshold, control the relay connected to any inverter leg to open, and control the relay connected to any inverter leg to close again within the set time period during the process of inputting the common-mode voltage signal to at least one inverter leg. Here, the voltage across the relay is detected by a sampling circuit, and the contact impedance of the relay is obtained by combining the voltage across the relay and the current flowing through the relay at present. If the contact impedance of the relay is still too large, control the relay to open for the next voltage difference cleaning, thereby further improving the cleaning effect on the oxidized relay.

[0010] In a possible implementation, the power converter outputs alternating current of N different phases. The controller is configured to control the frequency of the common-mode voltage signal to be N times the frequency of the alternating current output by any inverter leg, or M*N times the frequency of the alternating current output by any inverter leg, where M and N are positive integers. For example, when the power converter outputs three-phase alternating current of ABC, control the frequency of the common-mode voltage signal to be 3 times the frequency of the alternating current output by any inverter leg or M*3 times, such that the three-phase alternating current waves output by the power converter are mutually consistent, thereby ensuring the balance of the output currents of each phase of the power converter and avoiding excessive current on the switching tubes in the inverter legs.

[0011] In a possible implementation, the controller is configured to control the amplitude of the common-mode voltage signal to increase as the number of times the relay connected to any inverter leg is closed increases. During the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the relay when it is closed, and the voltage difference across the relay is proportional to the absolute value of the instantaneous value of the common-mode voltage signal input to the controller. Since the absolute value of the instantaneous value of the common-mode voltage signal is proportional to the amplitude of the common-mode voltage signal, by increasing the amplitude of the common-mode voltage signal, the absolute value of the instantaneous value of the common-mode voltage signal can be increased to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.

[0012] In a possible implementation, the controller is configured to control the amplitude of the common-mode voltage signal to increase as the operating temperature of the power converter during the previous grid connection increases. Since, when other conditions remain unchanged, the heat generated by the relay is proportional to its contact impedance. Therefore, the higher the operating temperature of the power converter during the previous grid connection, the controller increases the amplitude of the common-mode voltage signal, which can increase the absolute value of the instantaneous value of the common-mode voltage signal to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.

[0013] In a possible implementation, the common-mode voltage signal is a square wave. Compared with a triangular wave of the same frequency, when the square wave is used as the common-mode voltage signal input to the inverter leg, the change in the instantaneous value of the square wave in one period is smaller, ensuring that when cleaning the voltage difference across the oxidized relay, the voltage difference across the relay meets the cleaning requirements.

[0014] In a possible implementation, the common-mode voltage signal is a trapezoidal wave. Compared with a triangular wave of the same frequency, when the trapezoidal wave is used as the common-mode voltage signal input to the inverter leg, the change in the instantaneous value of the trapezoidal wave in one period is smaller, ensuring that when cleaning the voltage difference across the oxidized relay, the voltage difference across the relay meets the cleaning requirements. Description of the Drawings

[0015] Figure 1 is a schematic diagram of an application scenario of the power supply system provided by the present application;

[0016] Figure 2 is another schematic diagram of an application scenario of the power supply system provided by the present application;

[0017] Figure 3 is a schematic structural diagram of a power converter provided by the present application;

[0018] Figure 4 is another schematic structural diagram of a power converter provided by the present application;

[0019] Figure 5It is another structural schematic diagram of the power converter provided by this application;

[0020] Figure 6 It is a control schematic diagram for cleaning the relay of the power converter provided by this application;

[0021] Figure 7 It is another control schematic diagram for cleaning the relay of the power converter provided by this application;

[0022] Figure 8 It is another control schematic diagram for cleaning the relay of the power converter provided by this application;

[0023] Figure 9 It is a waveform schematic diagram of the superposition of the output of the power converter and the common - mode voltage signal provided by this application;

[0024] Figure 10 It is another waveform schematic diagram of the superposition of the output of the power converter and the common - mode voltage signal provided by this application. Specific embodiments

[0025] See Figure 1 , Figure 1 It is an application scenario schematic diagram of the power supply system provided by this application. The power supply system provided by this application may include a DC power supply and at least one power converter. Among them, the DC power supply is a photovoltaic module. Taking the power supply system including multiple power converters as an example, the DC terminal of the power converter is used to connect the photovoltaic module, and the AC terminals of the power converters are connected in parallel and then used to connect the power grid and the load. The power converter can invert and convert the direct current provided by the photovoltaic module and output the alternating current obtained after the inversion conversion to the power grid and the load for power supply.

[0026] In some feasible embodiments, the DC power supply is a storage battery, and the DC terminal of each power converter can be used to connect the storage battery. Please see Figure 2 , Figure 2 It is another application scenario schematic diagram of the power supply system provided by this application. Among the multiple power converters of the power supply system, the DC terminals of some power converters are used to connect the storage battery, and the AC terminals of the multiple power converters are connected in parallel and then used to connect the power grid and the load. Each power converter can invert and convert the direct current provided by the photovoltaic module or the storage battery and output the alternating current obtained after the inversion conversion to the power grid and the load for power supply. Here, the power converter connected to the photovoltaic module in the power supply system can be a photovoltaic inverter, and the power converter connected to the storage battery in the power supply system can be a current converter.

[0027] In Figure 1 or Figure 2In the application scenario of the power supply system shown, the power converter includes a controller, a power conversion circuit, and a relay. The input end of the power conversion circuit is connected to a photovoltaic module or an energy storage battery, and the output end of the power conversion circuit is connected to the power grid and the load through the relay. The above-mentioned controller is used to control the power conversion circuit to convert the received direct current into alternating current after the power converter is connected to the photovoltaic module or the energy storage battery, and control the relay to conduct to conduct the electrical connection between the power conversion circuit and the power grid and the load, so as to supply power to the power grid and the load. It should be understood that when other conditions remain unchanged, the heat generated by the relay is proportional to its contact impedance. Here, since the surface of the relay contacts in the power conversion circuit mostly adopts the process of plating inert metal, during the operation of the power converter, the following situations may occur: there may be corrosive gases outside the power converter and enter the inside of the power converter; or there may be some strongly chemically corrosive elements inside the power converter. The above corrosive factors such as corrosive gases and strongly chemically corrosive elements may cause the relay to oxidize, resulting in an increase in the contact impedance of the relay. When the power current passes through, the relay is prone to generate more heat and pose an overheating risk, thus affecting the normal operation and safety of the power converter. By applying a specific voltage across the oxidized relay, especially ensuring a certain voltage difference across the relay when it is closed, an electrochemical reaction occurs on the oxide layer of the relay, removing the oxide layer on the surface of the relay contacts and restoring its conductivity and switching performance. This cleaning method is also called voltage difference cleaning.

[0028] Currently, during the voltage difference cleaning process of the oxidized relay, a common-mode voltage signal such as a triangular wave is injected into the power converter while controlling the relay to conduct, so as to form a voltage difference across the relay, and the relay is closed during the period when there is a voltage difference across the relay, thereby performing voltage difference cleaning on the oxidized relay. However, during the injection of the common-mode voltage signal such as a triangular wave, the voltage difference between the output voltage of the power converter and the grid voltage may be zero. Due to the different conduction delays of the relay, the conduction action time of the relay is uncertain. Therefore, the voltage difference at the moment when the relay closes may be in a large voltage difference range or near zero crossing, and the effect of voltage difference cleaning cannot be guaranteed.

[0029] In order to improve the voltage difference cleaning effect on the oxidized relay, in the power converter provided in this application, the power converter includes a controller, at least one inverter bridge arm, and at least one relay, and each of the inverter bridge arms is connected to the power grid through at least one of the relays. For example, taking the power converter including one inverter bridge arm as an example, please refer to Figure 3 , Figure 3FIG. 0 is a schematic structural diagram of a power converter provided by the present application. The power converter includes an inverter bridge arm A and a relay Ka. The output end of the inverter bridge arm A is connected to the power grid through the relay Ka. In addition, the power converter includes a series-connected bus capacitor C1 and a bus capacitor C2, and the inverter bridge arm A is connected in parallel across the series-connected bus capacitor C1 and bus capacitor C2. Taking the power converter connected to a photovoltaic module as an example, the bus capacitor C1 is connected to the positive electrode of the photovoltaic module through the DC bus BUS+, and the bus capacitor C2 is connected to the negative electrode of the photovoltaic module through the DC bus BUS-. Before the power converter is connected to the power grid for operation, a differential pressure cleaning is performed on the relay connected to the power converter. For example, when the relay Ka is oxidized and needs to be cleaned, the controller ( Figure 3 not shown) in the power converter is used to control the inverter bridge arm A to convert the direct current from the two bus capacitors into alternating current and output it. During the process of controlling the inverter bridge arm A to convert the received direct current into alternating current and output it, a common-mode voltage signal is input to the inverter bridge arm A to change the instantaneous voltage value of the alternating current output by the inverter bridge arm A. Then, the controller is further used to control the relay Ka connected to the inverter bridge arm A to close within a set time period during the process of inputting the common-mode voltage signal to the inverter bridge arm A, so as to reduce the contact impedance of the relay Ka. Here, the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to a set threshold within the set time period. Here, since during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage differential across the relay when it is closed. And when the power grid voltage is stable, the voltage differential across the relay is directly related to the instantaneous voltage value of the alternating current output by the corresponding bridge arm. Therefore, the voltage differential across the relay is proportional to the absolute value of the instantaneous value of the common-mode voltage signal input by the controller. The controller controls the relay Ka connected to the inverter bridge arm A to close within the set time period of inputting the common-mode voltage signal. Since the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to the set threshold within the set time period and there is no zero-crossing point within the set time period, the input common-mode voltage signal can adjust the voltage magnitude at the output end of the inverter bridge arm A. The voltage differential across the relay Ka (i.e., the difference between the voltage at the output end of the inverter bridge arm A and the power grid voltage) within the set time period all meets the differential pressure cleaning requirements, avoiding the voltage differential across the relay Ka being too low when the relay Ka actually closes due to the conduction delay of the relay Ka, improving the reliability of the differential pressure cleaning of the relay Ka, reducing the contact impedance of the relay Ka, and avoiding overheating of the power converter.

[0030] In some feasible embodiments, the power converter includes a plurality of inverter bridge arms. Taking the power converter including three inverter bridge arms as an example, please refer to Figure 4 , Figure 4It is another structural schematic diagram of the power converter provided by this application. The power converter includes an inverter leg A, an inverter leg B, and an inverter leg C. The output terminals of the inverter leg A, the inverter leg B, and the inverter leg C are respectively used as the A-phase output terminal, the B-phase output terminal, and the C-phase output terminal of the power converter. The inverter leg A, the inverter leg B, and the inverter leg C in the power converter are respectively connected to the power grid through relays Ka, Kb, and Kc. In Figure 4 In the shown power converter, the controller ( Figure 4 not shown) in the power converter is used to control the inverter leg A, the inverter leg B, and the inverter leg C to convert the direct current from two bus capacitors into alternating current and output it. Similarly, before the power converter is connected to the power grid for operation, when Figure 4 any relay in the shown power converter oxidizes and needs to be cleaned, the controller is used to input a common-mode voltage signal to the inverter leg A, the inverter leg B, and the inverter leg C during the process of controlling the inverter leg A, the inverter leg B, and the inverter leg C to convert the received direct current into alternating current and output, so as to change the instantaneous voltage value of the alternating current output by each inverter leg. Then, the controller is further used to control the relay that needs to be cleaned in the power converter to close within a set time period during the process of inputting the common-mode voltage signal to the inverter leg A, the inverter leg B, and the inverter leg C, where the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to a set threshold within the set time period. Since the voltage difference across the relay satisfies the differential pressure cleaning requirement within the above-mentioned set time period, it is possible to avoid the situation that the voltage difference across the relay is too low when the relay actually closes due to the conduction delay of the relay, improve the reliability of differential pressure cleaning of the relay, reduce the contact impedance of the relay, and avoid overheating of the power converter.

[0031] Optionally, the output terminal of each inverter leg can also be connected to two serially connected relays. Please refer to Figure 5 , Figure 5 It is another structural schematic diagram of the power converter provided by this application. As Figure 5 shown, the output terminals of each inverter leg are all connected to two relays. The output terminal of the inverter leg A is connected to the relay Ka1 and the relay Ka2, the output terminal of the inverter leg B is connected to the relay Kb1 and the relay Kb2, and the output terminal of the inverter leg C is connected to the relay Kc1 and the relay Kc2. By increasing the number of relays connected to each inverter leg, it is possible to avoid the situation that some relays among the relays connected to any inverter leg fail and cannot be disconnected from the power grid. In Figure 5 In the shown power converter, the controller ( Figure 5 not shown) in the power converter is used to control the inverter leg A, the inverter leg B, and the inverter leg C to convert the direct current from two bus capacitors into alternating current and output. When Figure 5When any relay in the shown power converter oxidizes and needs to be cleaned, the controller is used to input a common-mode voltage signal to the inverter leg A, inverter leg B, and inverter leg C during the process of controlling the inverter leg A, inverter leg B, and inverter leg C to convert the received direct current into alternating current and output, so as to change the instantaneous voltage value of the alternating current output by each inverter leg. Taking the differential pressure cleaning of the relay Ka1 connected to the inverter leg A in the power converter as an example, the controller first controls the relay Ka2 connected in series with the relay Ka1 to be cleaned to close, and then controls the relay Ka1 to be cleaned in the power converter to close during a set period in the process of inputting the common-mode voltage signal to the inverter leg A, inverter leg B, and inverter leg C, where the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to the set threshold during the set period. Since the voltage differential across the relay Ka1 meets the differential pressure cleaning requirements during the above set period, it is possible to avoid the voltage differential across the relay Ka1 being too low when the relay Ka1 actually closes due to the conduction delay of the relay Ka1, improve the reliability of the differential pressure cleaning of the relay, reduce the contact impedance of the relay, and avoid overheating of the power converter.

[0032] In some feasible embodiments, the controller is used to control the relay connected to any inverter leg to close when the absolute value of the instantaneous value of the common-mode voltage signal rises from less than the set threshold to equal the set threshold. Please refer to Figure 6 , Figure 6 is a control schematic diagram of the relay cleaning of the power converter provided by this application. Taking the common-mode voltage signal input by the controller to the inverter leg in the power converter as a trapezoidal wave as an example, and taking the differential pressure cleaning of the relay connected to the inverter leg A in the power converter as an example, as Figure 6As shown, the common-mode voltage signal Ucm input by the controller to the inverter bridge arm in the power converter has the same phase and frequency as the alternating current output by the inverter bridge arm A. After the common-mode voltage signal Ucm is input, the alternating current output by the inverter bridge arm A changes from Ua1 to Ua2. Taking the maximum value of the common-mode voltage signal Ucm as the above-mentioned set threshold (alternatively, the above-mentioned set threshold can also be set lower than the maximum value of the common-mode voltage signal Ucm), the set period of the common-mode voltage signal Ucm is between t1 and t2, or can also be between t3 and t4. Among them, between t1 and t2, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold; between t3 and t4, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, and the direction is opposite to the direction of the common-mode voltage signal Ucm between t1 and t2. Taking the set period between t1 and t2 as an example, the controller is used to control the relay connected to the inverter bridge arm A to close at the moment when the absolute value of the instantaneous value of the above-mentioned common-mode voltage signal Ucm increases from less than the set threshold to just equal to the set threshold, that is, at time t1. Here, since the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold between t1 and t2, when the relay has a conduction delay, the set period between t1 and t2 can cover the time interval from when the controller controls the relay to close to when the relay actually closes, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, the common-mode voltage signal Ucm can adjust the voltage magnitude at the output end of the inverter bridge arm A, and the voltage difference across the relay meets the requirements of differential pressure cleaning, further improving the reliability of differential pressure cleaning of the relay.

[0033] In some feasible embodiments, if the conduction delay of the relay is relatively long, the controller is used to control the relay connected to any inverter bridge arm to close after the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration, and the set waiting duration is less than or equal to the duration of the set period. Again, taking the differential pressure cleaning of the relay connected to the inverter bridge arm A in the power converter as an example, please refer to Figure 6 again. If the conduction delay of the relay is relatively long, for example, the time interval from when the controller controls the relay to close to when the relay actually closes is equal to the above-mentioned set period, then it can be controlled that the relay connected to the inverter bridge arm A closes at time t5 after time t1, that is, after the absolute value of the instantaneous value of the above-mentioned common-mode voltage signal Ucm increases from less than the set threshold to just equal to the set threshold and after an interval of the set waiting duration Ts. The actual closing moment of the relay is between another set period, that is, between t3 and t4, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, the common-mode voltage signal Ucm can adjust the voltage magnitude at the output end of the inverter bridge arm A, and the voltage difference across the relay meets the requirements of differential pressure cleaning, further improving the reliability of differential pressure cleaning of the relay.

[0034] In some feasible embodiments, the common-mode voltage signal input by the controller to the inverter leg in the power converter may be a square wave. Please refer to Figure 7 , Figure 7 which is another control schematic diagram of the relay cleaning of the power converter provided in this application. Similarly, taking the differential pressure cleaning of the relay connected to the inverter leg A in the power converter as an example, as Figure 7 shown, the phase and frequency of the common-mode voltage signal Ucm input by the controller to the inverter leg in the power converter are the same as those of the alternating current output by the inverter leg A. After the input of the common-mode voltage signal Ucm, the alternating current output by the inverter leg A changes from Ua1 to Ua2. Taking the maximum value of the common-mode voltage signal Ucm as the above-mentioned set threshold (or, the above-mentioned set threshold can also be set lower than the maximum value of the common-mode voltage signal Ucm), the set period of the common-mode voltage signal Ucm is between t1 and t2, or can also be between t2 and t3. Among them, between t1 and t2, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold; between t2 and t3, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, and the direction is opposite to the direction of the common-mode voltage signal Ucm between t1 and t2. Similarly, the controller is used to control the relay connected to the inverter leg A to close at the moment t1 when the absolute value of the instantaneous value of the common-mode voltage signal Ucm rises from less than the set threshold to just equal to the set threshold. When there is a conduction delay in the relay, the set period between t1 and t2 can cover the time interval from when the controller controls the relay to close to when the relay actually closes, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, and the common-mode voltage signal Ucm can adjust the voltage magnitude at the output end of the inverter leg A. If the conduction delay of the relay is relatively long, such as the time interval from when the controller controls the relay to close to when the relay actually closes is equal to the above-mentioned set period, then after the absolute value of the instantaneous value of the common-mode voltage signal Ucm rises from less than the set threshold to just equal to the set threshold and after an interval of the set waiting duration Ts, that is, at the moment t3 after t1, the controller controls the relay connected to the inverter leg A to close. The actual closing moment of the relay is between another set period, that is, between t2 and t3, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, the common-mode voltage signal Ucm can adjust the voltage magnitude at the output end of the inverter leg A, and the voltage differential pressure across the relay meets the differential pressure cleaning requirements, further improving the reliability of the differential pressure cleaning of the relay.

[0035] In some feasible embodiments, the common-mode voltage signal is a periodically varying alternating current signal, and the controller controls the relay to close multiple times to perform differential pressure cleaning on the relay. Specifically, the controller is configured to control the relay connected to any inverter bridge arm to close once each time the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration. Wherein, the set waiting duration increases after each time the controller controls the relay connected to any inverter bridge arm to close, and the set waiting duration is less than or equal to the duration of the set period. Please refer to Figure 8 , Figure 8 FIG. Figure 8 is another control schematic diagram of relay cleaning for the power converter provided in the present application. Taking the common-mode voltage signal input by the controller to the inverter bridge arm in the power converter as a periodically varying square wave as an example, and taking the differential pressure cleaning of the relay connected to the inverter bridge arm A in the power converter as an example, as Figure 8As shown, taking the maximum value of the common-mode voltage signal Ucm as the set threshold, the set time period of the common-mode voltage signal Ucm includes t1 to t2, t2 to t3, t3 to t4, t4 to t5, t5 to t6, and t6 to t7, etc. The controller is used to control the relay connected to the inverter bridge arm A to close once every time the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold and after a set waiting duration. Taking the initial value of the above set waiting duration as 0 and the duration incremented each time as T0 / 6 as an example, T0 is the duration for which the common-mode voltage signal reaches the maximum value in each cycle. First, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold at the moment t1, and the controller controls the relay connected to the inverter bridge arm A to close once at the moment t1, and then the controller controls the relay to disconnect. The absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold at the moment t2, the set waiting duration is incremented by T0 / 6, and at the moment t8, which is T0 / 6 after the moment t2, the controller controls the relay connected to the inverter bridge arm A to close, and then the controller controls the relay to disconnect. The absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold at the moment t3, the set waiting duration is incremented by T0 / 6, and at the moment t9, which is 2T0 / 6 after the moment t3, the controller controls the relay connected to the inverter bridge arm A to close, and then the controller controls the relay to disconnect. Similarly, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold at the moments t4, t5, and t6, and the controller controls the relay connected to the inverter bridge arm A to close at the moment t10, which is 3T0 / 6 after the moment t4, at the moment t11, which is 4T0 / 6 after the moment t5, and at the moment t12, which is 5T0 / 6 after the moment t6, respectively. Here, by controlling the relay connected to the inverter bridge arm A to close once every time the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold and after a set waiting duration, and the above set waiting duration is incremented each time the controller controls the relay to close, it is ensured that in multiple controls of the relay closing by the controller, the moment of each control of the relay closing is at a different position in each set time period. For example, as Figure 8 shown, the moment when the controller first controls the relay to close is at the starting moment (i.e., t1) in the set time period t1 to t2, and as the number of closings increases, the moment when the controller controls the relay to close gradually moves backward. For example, at the middle moment (i.e., t10) in the set time period t4 to t5 and at the end moment (i.e., t12) in the set time period t6 to t7. The moment when the controller controls the relay to close each time is at a different position in each set time period, which can avoid the problem that the common-mode voltage signal Ucm is at zero at the actual closing moment of the relay due to the uncertain conduction delay of the relay, ensuring that when the relay actually closes, the absolute value of the instantaneous value of the common-mode voltage signal Ucm is equal to the set threshold, and the common-mode voltage signal Ucm can adjust the voltage magnitude at the output end of the inverter bridge arm A, ensuring that the voltage difference across the relay meets the differential pressure cleaning requirements.

[0036] In some feasible embodiments, the power converter outputs alternating current with N different phases, and the controller is configured to control the frequency of the common-mode voltage signal to be N times, or M*N times, where M and N are positive integers, the frequency of the alternating current output by any inverter leg. For example, when the power converter outputs alternating current with three different phases, the power converter may be the power converter shown above Figure 4 or Figure 5 . The power converter outputs phase-A alternating current, phase-B alternating current, and phase-C alternating current, and the phase difference between two-phase alternating current is 120 degrees. Refer to Figure 9 . Figure 9 is a schematic waveform diagram of the output of the power converter provided in this application superimposed with the common-mode voltage signal. If the common-mode voltage signal Ucm input by the controller to the inverter leg in the power converter is a square wave, and the phase and frequency of the common-mode voltage signal Ucm are the same as those of the alternating current output by inverter leg A. After inputting the common-mode voltage signal Ucm, the alternating current output by inverter leg A changes from Ua1 to Ua2, the alternating current output by inverter leg B changes from Ub1 to Ub2, and the alternating current output by inverter leg C changes from Uc1 to Uc2. The waveforms of the above Ua2, Ub2, and Uc2 are different from each other. It can be seen that when the power converter outputs three-phase alternating current of ABC and the frequency of the common-mode voltage signal is controlled to be 1 times the frequency of the alternating current output by any inverter leg, it will cause the waveforms of the alternating current output by each inverter leg in the power converter to be inconsistent. It should be understood that when the waveforms of the alternating current output by each inverter leg in the power converter are inconsistent, it will cause the output current of each phase of the power converter to be unbalanced, which may cause the output current of a certain phase of the power converter to be too large. If the output current of a certain phase of the power converter is too large, the switching device in the corresponding leg may bear too large a current, increasing the switching device loss. Please refer to Figure 10 . Figure 10This is another waveform schematic diagram of the superposition of the output of the power converter and the common-mode voltage signal provided by this application. If the common-mode voltage signal Ucm input by the controller to the inverter bridge arm in the power converter is in the same phase as the alternating current output by the inverter bridge arm A, and the frequency is 3 times that of the alternating current output by the inverter bridge arm A, after inputting the common-mode voltage signal Ucm, the alternating current output by the inverter bridge arm A changes from Ua1 to Ua2, the alternating current output by the inverter bridge arm B changes from Ub1 to Ub2, and the alternating current output by the inverter bridge arm C changes from Uc1 to Uc2. It can be seen that when the power converter outputs three-phase alternating current ABC and the frequency of the controlled common-mode voltage signal is 3 times that of the alternating current output by any inverter bridge arm, the three-phase alternating current waves output by the power converter are consistent with each other, thereby ensuring the balance of the output current of each phase of the power converter and avoiding excessive current on the switching tubes in the inverter bridge arm. Optionally, the controller can also control the frequency of the common-mode voltage signal Ucm to be M*3 times that of the alternating current output by the inverter bridge arm A, such as 6 times, 9 times, etc. And when the frequency of the controlled common-mode voltage signal Ucm is M*3 times that of the alternating current output by the inverter bridge arm A, the differential pressure cleaning process of the relay by the controller is similar to the scenario with the same frequency as described above, which will not be elaborated here.

[0037] In some feasible embodiments, after the controller controls the relay connected to any inverter bridge arm to close, the controller is used to, if the contact impedance of the relay connected to any inverter bridge arm is greater than the set impedance threshold, control the relay connected to any inverter bridge arm to open, and control the relay connected to any inverter bridge arm to close again within a set period during the process of inputting the common-mode voltage signal to any inverter bridge arm. Specifically, taking the differential pressure cleaning of the relay Ka1 connected to the inverter bridge arm A in the power converter shown above as an example, during the process of the controller controlling the inverter bridge arm A to convert the received direct current into alternating current and output it, the common-mode voltage signal is input to change the instantaneous voltage value of the alternating current output by the inverter bridge arm A. Then, the controller is also used to control the relay Ka connected to the inverter bridge arm A to close within a set period during the process of inputting the common-mode voltage signal to the inverter bridge arm A to reduce the contact impedance of the relay Ka. When the relay Ka is closed, the controller obtains the contact impedance of the relay Ka. For example, the voltage across the relay is detected through a sampling circuit, and the contact impedance of the relay Ka is obtained by combining the voltage across the relay and the current flowing through the relay at present (i.e., the output current of phase A of the power converter). If the contact impedance of the relay Ka is still too large, such as greater than the set impedance threshold, the relay Ka is controlled to open for the next differential pressure cleaning, so as to further improve the cleaning effect on the oxidized relay. Figure 3 This is another waveform schematic diagram of the superposition of the output of the power converter and the common-mode voltage signal provided by this application. If the common-mode voltage signal Ucm input by the controller to the inverter bridge arm in the power converter is in the same phase as the alternating current output by the inverter bridge arm A, and the frequency is 3 times that of the alternating current output by the inverter bridge arm A, after inputting the common-mode voltage signal Ucm, the alternating current output by the inverter bridge arm A changes from Ua1 to Ua2, the alternating current output by the inverter bridge arm B changes from Ub1 to Ub2, and the alternating current output by the inverter bridge arm C changes from Uc1 to Uc2. It can be seen that when the power converter outputs three-phase alternating current ABC and the frequency of the controlled common-mode voltage signal is 3 times that of the alternating current output by any inverter bridge arm, the three-phase alternating current waves output by the power converter are consistent with each other, thereby ensuring the balance of the output current of each phase of the power converter and avoiding excessive current on the switching tubes in the inverter bridge arm. Optionally, the controller can also control the frequency of the common-mode voltage signal Ucm to be M*3 times that of the alternating current output by the inverter bridge arm A, such as 6 times, 9 times, etc. And when the frequency of the controlled common-mode voltage signal Ucm is M*3 times that of the alternating current output by the inverter bridge arm A, the differential pressure cleaning process of the relay by the controller is similar to the scenario with the same frequency as described above, which will not be elaborated here.

[0038] In some feasible embodiments, when the controller controls the relay to close multiple times for differential pressure cleaning of the relay, the controller controls the amplitude of the common-mode voltage signal to increase as the number of times the relay is controlled to close increases. Specifically, the expression of the common-mode voltage signal input by the controller to the power converter is:

[0039] g(t) = A m *δ(N g *2πf g *t)

[0040] where A m is the amplitude of the common-mode voltage, and this amplitude of the common-mode voltage can be set to a positive value or a negative value, f g is the fundamental frequency, δ(t) is a pulse function or a trapezoidal wave function, and N g is the fundamental frequency increase coefficient. It should be understood that during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage differential across the relay when it is closed, and the voltage differential across the relay is proportional to the absolute value of the instantaneous value of the common-mode voltage signal input by the controller. After the controller controls the relay to close multiple times for differential pressure cleaning of the relay, if the contact impedance of the relay is still too high, it means that when the relay is closed, the input common-mode voltage signal cannot make the voltage differential across the relay reach the cleaning requirement. Since the absolute value of the instantaneous value of the common-mode voltage signal is proportional to the amplitude of the common-mode voltage signal, therefore, by increasing the amplitude of the common-mode voltage signal, the absolute value of the instantaneous value of the common-mode voltage signal can be increased to increase the voltage differential across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.

[0041] In some feasible embodiments, the controller is configured to control the amplitude of the common-mode voltage signal to increase as the operating temperature at the previous grid connection of the power converter increases. Since, when other conditions remain unchanged, the heat generated by the relay is proportional to its contact impedance. Therefore, before cleaning the oxidized relay, obtain the operating temperature at the previous grid connection of the power converter. If the temperature is higher, the contact impedance of the relay is higher and the degree of oxidation is greater. For example, by setting a temperature detection device on the corresponding relay, this temperature detection device can be set at the wiring terminal of the relay or beside the contact of the relay, and the heating temperature when the relay is closed is obtained through the temperature detection device to obtain the operating temperature when the power converter is grid-connected. As the operating temperature at the previous grid connection of the power converter is higher, the controller increases the amplitude of the common-mode voltage signal, which can increase the absolute value of the instantaneous value of the common-mode voltage signal to increase the voltage differential across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.

Claims

1. A power converter, characterized in that, The power converter includes a controller, at least one inverter bridge arm, and at least one relay. The output end of each inverter bridge arm is connected to the power grid through at least one of the relays; The controller is configured to control the at least one inverter bridge arm to convert the received direct current into alternating current and output it; The controller is configured to, during the process of controlling the at least one inverter bridge arm to convert the received direct current into alternating current and output it, input a common-mode voltage signal to the at least one inverter bridge arm to change the instantaneous value of the voltage of the alternating current output by the at least one inverter bridge arm; The controller is further configured to, within a set time period during the process of inputting the common-mode voltage signal to the at least one inverter bridge arm, control the relay connected to any one of the at least one inverter bridge arms to close, where the absolute value of the instantaneous value of the common-mode voltage signal is greater than or equal to a set threshold within the set time period.

2. The power converter according to claim 1, wherein, The controller is configured to, when the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to equal to the set threshold, control the relay connected to any one of the inverter bridge arms to close.

3. The power converter according to claim 1, characterized in that, The controller is configured to, after the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration, control the relay connected to any one of the inverter bridge arms to close, and the set waiting duration is less than or equal to the duration of the set time period.

4. The power converter according to claim 1, characterized in that The common-mode voltage signal is an alternating current signal with periodic variation. The controller is configured to, after the absolute value of the instantaneous value of the common-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration each time, control the relay connected to any one of the inverter bridge arms to close once; Wherein, the set waiting duration increases after each time the controller controls the relay connected to any one of the inverter bridge arms to close, and the set waiting duration is less than or equal to the duration of the set time period.

5. The power converter according to any one of claims 1-4, characterized in that, After the controller controls the relay connected to any one of the inverter bridge arms to close, if the contact impedance of the relay connected to any one of the inverter bridge arms is greater than a set impedance threshold, the controller is configured to control the relay connected to any one of the inverter bridge arms to open and control the relay connected to any one of the inverter bridge arms to close again within a set time period during the process of inputting the common-mode voltage signal to the at least one inverter bridge arm.

6. The power converter according to any one of claims 1-5, characterized in that, The power converter outputs alternating current with N different phases. The controller is configured to control the frequency of the common-mode voltage signal to be N times the frequency of the alternating current output by any one of the inverter bridge arms, or M*N times the frequency of the alternating current output by any one of the inverter bridge arms, where M and N are positive integers.

7. The power converter according to claim 5, characterized in that, The controller is configured to control the amplitude of the common-mode voltage signal to increase as the number of times the relay connected to any one of the inverter bridge arms is closed increases.

8. The power converter according to any one of claims 1-7, characterized in that, The controller is configured to control the amplitude of the common-mode voltage signal to increase as the operating temperature of the power converter during the previous grid connection increases.

9. The power converter according to any one of claims 1-8, characterized in that, The common-mode voltage signal is a square wave.

10. The power converter according to any one of claims 1-8, characterized in that, The common-mode voltage signal is a trapezoidal wave.

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